Much is now known about not only the inhibitory effect of APOBEC3G, and other related APOBEC3 proteins, on HIV-1 replication but also the mechanism of APOBEC3G neutralization by HIV-1 Vif. However, the APOBEC3 protein family must be evolutionarily conserved in humans for reasons unrelated to HIV-1 infection, as this virus only entered the human population relatively recently. In this grant, we focus on understanding the role and mechanism of action of APOBEC3G, and related proteins, in controlling not only retroviruses distinct from HIV-1, but also LTR retrotransposons, a key class of endogenous genomic parasites. In the case of primate foamy virus (PFV), the prototype of a ubiquitous family of non-pathogenic retroviruses, we present evidence for a novel functional homolog of Vif, termed Bet that specifically binds to APOBEC3G in vivo. In the absence of a functional Bet protein, PFV is highly susceptible to APOBEC3G, which packages into PFV virions and induces editing of nascent proviruses. How APOBEC3G packages into PFV virions is unclear, as PFV Gag does not contain sequences related to the HIV-1 nucleocapsid zinc fingers implicated in virion packaging of APOBEC3G. In the case of murine leukemia virus (MLV), a prototypic oncoretrovirus, human APOBEC3G is highly inhibitory while murine APOBEC3 is far less so and is excluded from MLV virions. We will seek to identify the determinants regulating MLV virion incorporation of these proteins and their ability to bind MLV Gag. Finally, we have obtained exciting new data showing that human APOBEC3G can specifically block retrotransposition of Tyl in yeast cells. We will seek to identify the mechanism underlying this inhibition and, most importantly, will attempt to use genetic approaches to identify yeast proteins that facilitate inhibition of Tyl retrotransposition by APOBEC3G as a way of potentially identifying analogous APOBEC3G co-factors in HIV-1 infected human cells. Together, these data should shed significant light on the role and mechanism of action of APOBEC3G and related proteins and pave the way for future research aimed at enhancing or protecting APOBEC3G function and thereby inhibiting HIV-1 replication in vivo.